Soundstage

There can be a bad connection inside one of the connected devices.

Get a VOM and clip leads. Measure conductor and shield end to end on the interconnects while vigorously wiggling the connectors. Resistance should remain ZERO.

Repeat with each connector measuring between conductor and shield. Do both ends of the cable. Resistance should remain infinite.

If the above are true, problem is internal. DO NOT wiggle connectors to a power amp. You run the risk of dumping full power into the speakers. Use a dummy load. 

Hate to say it but maybe your hearing?  I started having a problem over this past winter where the right side seemed slightly muted at times.  Soundstage seemed shifted to the left.  Barely noticeable but frustrating.  It took maybe a month or so to realize that I had some tinnitus starting in the right ear.  Nothing wrong with my gear/setup.  

How loud do you listen?  Could be after a few hours you start having auditory issues

Hello,
You and another person talked about having the same issue. I do like MC’s answer. People forget what their system sounds like. It might be your capacitors. Uneven sound, cracking, and inconsistent times. I am not sure of what your pre/ amp is but caps do go bad and can give off inconsistent sound signatures. Power it up and listen. If it is a tube product it could be a tube. Roll the tubes from L to R and R to Left. If it is solid state switch the IC at one of the ends. Also there can be caps in your speaker crossovers. Switch the speaker wires too to see. Or try the speakers on another device. Sounds like a cap or tube problem.